If your car is being repaired due to a car accident which you were not at fault for, you should see if the at-fault party's insurance company has a rental car allowance during the time your car is in the auto shop being repaired.

Instead, if you were at fault in an accident and are putting a claim through your insurance company, you should see if you have rental car coverage. Typically there is optional rental car coverage that you can obtain through your insurance company. Your insurance company can tell you if you have this optional coverage and if so what your limits are with it.

Now if you have not been in an accident but are getting repairs done for other reasons, or the car is in with the mechanic for regular maintenance, the optional rental car coverage would not cover a rental car during this period.

Rental car coverage by an insurance company is typically worded in a way to say that that the coverage is available only due to a covered loss. So if your vehicle has not been involved in a situation described as a "covered loss" than your insurance company would not pay for or reimburse you for a rental car.