What solution is: Check out venv\pyvenv.cfg and provide a valid path to the basic python installation.
What has most probably happend:
After reinstalling your OS, you have no base python interpreter reinstalled or you have installed it at different location than before. Thus your virtual environment fails to locate the python installation. Virtual environment implies that all libraries and settings are isolated from other projects. It does not provide an isolated python installation. You still need your base python that had been used for venv creation.
My case: I have a project in a network share and try accessing it from different computers. The base python paths depend on the very PC. The solution above works fine for me. Unfortunatelly, I need to update pyvenv.cfg depending on the PC in use.
P.S. I believe that there is an environment variable to overide the venv config value. I only tried to set PYTHONPATH=C:\Anaconda3\envs\python37 on Windows and then to activate venv. It did no effect and I gave it up.