Here is a video showing how to do a "soft reset" on HMD phones...
Soft reset, should fix most problems:
Video on doing a soft reset
If it isn't clear to you, HERE is a web page that explains the same process with pictures.
"Completely messed it up" covers a lot of possible problems, forgot pin, filled the memory with too many apps, I don't know what... So this may NOT fix it. If So, I'd need more information as to EXACTLY how it got that way and maybe I could help you fix it short of the LAST RESORT which would be a "Hard" reset, but that LOSES al your personal data and apps by putting it back in an out-of-the-box state, where you have to set it up (basically as a new phone) NOT the preferred solution...
This Web Page goes over how to do a HARD reset (factory reset) and This Video goes over one way to do the Hard reset.
Good luck.
Again before taking the FINAL RESORT you might tell us more so maybe you don;t have to do the factory reset.
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