With sufficient worldbuilding, your creatures could be affected by
Electric Imbalance Syndrome (EIS)
Similar to amflare's answer, but differing in that Electric Imbalance Syndrome is biological in nature instead of affecting whatever happens to be unlucky enough to be nearby when the sun decides to screw everyone over.
Gravity is indeed a much weaker force than electric force. How much weaker?
Newtonian gravity is modeled by the equation $F_g = G\frac{Mm}{r^2}$. Electric force is modeled similarly by $F_e = \frac{1}{4\pi\epsilon_0}\frac{q_1q_2}{r^2}$.
We can compare them, quite simply, by dividing one by the other:
$$\frac{F_e}{F_g} = \frac{1}{4\pi\epsilon_0}\frac{q_1q_2}{r^2}\frac{r^2}{GMm} = \frac{q_1q_2}{4\pi\epsilon_0GMm}$$
Substituting in all the constants and canceling $r^2$, this comes to
$$\frac{F_e}{F_g} = \left(1.35 \times 10^{20} \frac{kg^2}{C^2}\right)\frac{q_1q_2}{Mm}$$
If we choose two protons to use as our basis for comparison, the charge of a proton is $1.60 \times 10^{-19} C$ and the mass of a proton is $1.67 \times 10^{-27} kg$, so plug all that in and we find that the electric force between them is
$$\frac{F_e}{F_g} = \left(1.35 \times 10^{20} \frac{kg^2}{C^2}\right)\frac{(1.60 \times 10^{-19} C)^2}{(1.67 \times 10^{-27} kg)^2} = 1.24 \times 10^{36}$$
So the electric force between them is roughly $10^{36}$ times stronger than the gravitational force, if I did my math right.
So perhaps your planet is composed largely of minerals with a low conductivity (insulators) and contains positive or negative charges distributed uniformly within. This will result in a near-uniform electric field directed toward or away from the planet. This way, any charges in the creatures' bodies will cause them to be attracted to or repelled from the planet. A very delicate balance of positive and negative electric charges is then necessary for your species to survive.
Perhaps these charges will be balanced through diet, like vitamins. Eat too much of foods with one charge and find yourself being pulled "heavily" toward the ground. Eat too much of the other and risk flying off into the sky. When perfectly balanced, the creatures experience only gravity, as we do on Earth. Perhaps optimal balance should keep the creature a little "light" on its feet to reduce wear on the legs, feet and knees.
Natural excretion will no doubt regulate these charges as the majority charge will be repelled and the minority charge attracted. Foods that reinforce the electric imbalance would also be difficult to pick up or eat. But perhaps sufficiently poor diet could outpace these natural processes or simply provide no charges to counteract the effect of the majority charge, or perhaps some illness causes' the creature's antibodies to forcefully expel either positive or negative charges.
EIS could theoretically function in either variety, the kind that pulls the victim to the ground or the kind that flings the victim into the air. These could be known as Positive EIS and Negative EIS, depending on the charge of the planet itself. Proper worldbuilding could make one much more rare than the other. Perhaps negatively-charged food is uncommon or unpleasant on this positively-charged planet. In such event, Positive EIS is the common condition where a creature's body tends away from the planet, eventually sending them off into the clouds.
There is little hope for someone whose fate lies in the sky, as both gravity and electric force scale with distance squared. Once a creature's outward-directed electric force exceeds its inward-directed gravitational force, it always will regardless of how far away the creature's body moves. Those who are fortunate enough to be indoors at such time may be saved, or perhaps someone surrounded by quick-acting friends might as well.
If these charges are to stay within the body, which they must if this entire world is to work, the creature's skin must also be an insulator. As such, one potential remedy is to simply break the skin at the top of the head and allow the outward-tending charges to escape on their own.
Another remedy might be pharmaceutical charge tablets that maintain one's charge when used as directed by your doctor. Do not use these tablets if you are nursing, pregnant or may become pregnant. Side effects include bleeding, shortness of breath, acute pain and difficulty excreting. Contact your doctor if you experience an... well, you know the rest.