Upgraded to EE 4.2.0 from EE 3.5.15 and now can't login to the backend

Welcome YellowDigital. Sorry you’re having difficulties. It’s hard to say what issue you might have as you’ve not given any details about what you’ve tried, what happens, and so on. That said, this particular forum is for Development and Programming questions, so you may wish to put in a support ticket. If you have a valid license and have not used our support before, you are eligible for 3 months of free escalated support with a direct line to our engineers for assistance.