A Year or Two
So when this happens you will have some areas of the globe that will become massive centers of death. Places like New York City simply have too high of a population density. Without technology, people cannot get out of NYC and food cannot get in fast enough. However, you will have some places that will not be affected much in terms of life / death. In certain regions farming has still not become very industrialized.
For the focus of this answer, I am going to look at a college town, where the school has a decent engineering department.
Luckily, the national guard and/or military has enough people that don't desert that they are able to take charge and impose order. They have problems with bandits of course, but not enough to impact tech recovery.
However, food is going to be a problem. In the first two days everyone was grilling out and sharing with their community the massive amount of meat and other frozen goods that they have no way of eating before they go bad. Canned goods and non-perishables will last a few months, but long term solutions are needed. Luckily this is a small college town. There are not miles of suburbs to pass through.
It does not take people long to march out to the farm fields outside of town. While the machinery to harvest the food is gone, the crops are still growing. Most of the population will need to pitch in to help with the harvest, and the menu will be much more limited to what they are used to, but food is ensured for at least the next year.
In the automotive mechanics lab, a class was in the middle of stripping down engines. The individual parts are spread across tables and fell below the complexity level that was taken away. They are quickly taken away. The supply yard for the local power company provides us will all of the copper wire we could ever want. Without cars, there is plenty of gasoline available to use. Within a week of things getting organized, they have crude gasoline generators.
Some physics teachers remembered that they have some microwave horns in a supply closet for a lab that they have the undergrads do. It takes a bit of finagling, but they get a microwave built a bit later. Not necessary, but it brings people comfort. Raiding the optics lab for lenses, grabbing some loose electric motors (or building some if they all went away, its not too bad), and going to the old movie theater that never upgraded to digital, a month or two later someone has cobbled together a projector. There is no sound, wiring the speakers will take time, but the people are pacified watching a silent film version of Star Wars. In fact, they seem to enjoy adding in their own lines.
In the meantime, going to junk yards they have built more motors. Some have been mounted to the 4-wheel bike frame student projects in the engineering building, making light weight vehicles. It will take a bit more time to build something from the rusted hollow frames of abandoned cars, but they will work that steel, since all the tanks for welding are still around. These will be helpful in plowing the fields for the next planting.
Within two years, with electricity and lighting restored, food secured, and some semblance of old life restored, a few people assemble a crystal radio from a kit that was sitting in the old abandoned radio shack. More are then built from the parts in the electronics lab at the university. Radio communication at least within this town is restored.
At this point they begin considering what is outside their borders. There are many bandit tribes. Survivalists with their stock piles of weapons are on much more even footing with the military without their tech. Half-starved refugees are nearly a constant sight. Perhaps they will send a convey out towards the hoover dam. If the structure was left, someone probably managed to build some sort of turbine and generator system inside it.
Things are still precarious, survival is not guaranteed, but for the first time in a long while, people of this town feel genuine hope.
