Git pull / push - unable to access HTTPS, SSL routines seem to be down

Updating TortoiseGit and GCM didn't help me, but updating Git itself did, as per @Frederic's advice in comments.

https://git-scm.com/download/win

To make sure the new version of Git installs properly and doesn't conflict with previous installations (it might, if you used TortoiseGit's, because it would use different folders and mess with PATH variable), remove the existing Git installation before installing the updated Git. Might also need to install with administrator rights.

You're likely running into an incompatibility with GitHub's deprecation of weak SSL encryption protocols:

Weak cryptographic standards removal notice

The solution will vary, but for Windows you likely need to upgrade the Git credential manager to 1.14.0

https://github.com/Microsoft/Git-Credential-Manager-for-Windows/releases/tag/v1.14.0