Someone took over my PC

Hi everyone,
I left my pc downloading files overnight and meanwhile someone outside took over and searched on it, opening financial files, password managers (unsuccessfully).
Do you have any tips on what I should do to check and secure my PC.
PS Ubuntu Budgie 22.04 is updated.

Kind of a large question.

Was there physical access to the computer i.e. you left the computer unlocked and the person just used the open session?

Or did the person login remotely? If so from where? I.e. on the same network or was it from the internet? What is the security you use on your network and network devices such as routers and switches? What security do you use for wifi?

Thanks David for responding.
It is remote access.
I rent an apartment with wifi included. The wifi module is provided by the Internet provider MTS and it is a Huawei HG630.
And yes the password is quite simple and it may be one of my neighbors who accesses it. So I don’t know what the security level is.
I’m gonna check and change it right away (when I will find how to do it).

My advise for connecting to a wifi connection you cannot control is to use a VPN. Invest in a good one offering the openvpn standard.

Thus your network connection will be secure.

Are you sure that you have turned off remote access? HG630 is what MTS does use but from the top of the router could be probably using the default username and password. Aside from that, make sure that you are connecting to the internet via secured network.

Also, make sure you did left software such as AnyDesk or TeamViewer in seamless mod.