Updated kernel and now won't boot

Hi everyone. I was having issues with Bluetooth so decided to update the kernel from 5.4 lt to 5.9 to see if it fixed the issue. Now it won’t even boot. Just black screen and loads of white text. No idea what to do. And I’m pretty new to Linux

Any ideas?? Pretty stuffed without my laptop

You should still have the other one installed.
You should be able to choose the other kernel at boot time from the menu - if you did just install the new 5.9 and didn’t also remove the 5.4
which mhwd-kernel won’t let you do, AFAIK