When I pressed on the ruler tool(accidentally), and pressed on the artwork I made by accident, I went back to the brush tool and tried to edit my artwork and it froze, it didn't want to edit anything. I got worried so I tried to save it as .PNG and .KRA but it said that it couldn't save. Reproducible: Sometimes I couldn't save or draw on it anymore.
Hi, I'm not sure what happened, and I'm afraid that there's not enough information to figure out what you did to trigger this bug exactly. You should always be able to save to .kra... However, Krita 3.0 has been released last month, with a host of bug fixes and I would encourage you to give the new version a try.