I've been running Ubuntu 20.04 with plasma installed for a while now and i've encountered this weird issue where Plasma will lock up after a while when i'm using it. I've tried other desktop environments and it only happens on plasma.

It first starts as an inability to change windows, either with alt tab or by clicking the task manager. Notifications will play sound but won't show up in the notification area. After a minute or two it stops recognizing keyboard inputs or mouseclicks (i can still move the mouse cursor around though)
Only fix is to hard-reset my pc.

Any idea what this could be?
I could switch back to cinnamon but i like plasma a lot and i don't want to have to leave it behind due to this.
Any help is appreciated.

There are a lot of possible culprits, starting from a desktop effect over graphic card issues to a thread running wild using up all the memory, or any changes you might have made to the default installation (themes, additional desktop effects, etc)
Since you are on Ubuntu 20.04 you should have Plasma 5.19.5, up-to-date with the latest security fixes (there have been quite a few lately). There is always the possibility to upgrade to Ubuntu 20.10 which has a newer Plasma version. In any case, you should make sure you have all necessary packages, installing the package kubuntu-desktop should ensure a complete installation.
Please also provide information about your system, notably the graphic system you use (including which driver if you have a NVIDIA card) else there is really not much we can do other than guessing.

There is a quite easy way to restart Plasma without logging out:
In a shell (Ctrl+Alt+F2 is a handy one) type the following:

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killall plasmashell

go back to the previous shell (Ctrl+Alt+F1) and in a konsole type:

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plasmashell --replace &

Thanks for the help! I tried this same question in the AskUbuntu forum and never got an answer.

This worked, i guess plasma just needed to reinstall.

I have the same problem, but on 20.10, my current plasma ram usage in Sysytem monitor is 6gb... And sometimes the desktop just stops responding especially after being afk, sometimes I am not even able to change to tty I wonder is there a solution for this, how do i debug it? restarting plasma every few hours is a problem.

That is weird, did you try to rule out any of the possible culprits I mentioned above? I have never seem RAM spikes on my system since it was installed with 20.10
I would suspect a Desktop Effect to be the most likely culprit, followed by a graphic card driver issue. Since I never used a NVIDIA card (Intel and AMD only), but have heard so many problems with proprietary drivers, I wouldn't be surprised that this is the area to explore...