This is a common question I've seen being asked on Why krita suddenly stopped saving your work?
here are some possible solutions to your problem

If the message is “File not found. Check the file name and try again.”, you probably have Controlled Folder Access enabled.

1. Select Start > Settings .
2. Choose Update & security > Windows Defender.
3. Select Open Windows Defender Security Center.
4. Select Virus & threat protection, and then choose Virus & threat protection settings.
5. Under Controlled folder access, turn it on or off.

source: https://docs.krita.org/en/KritaFAQ.html#windows-krita-cannot-save

If Krita still isn't saving your file then follow these instructions instead...

1. Use the Windows Defender Security app to allow specific apps
Open the Windows Security by clicking the shield icon in the task bar or searching the start menu for Defender.

2. Click the Virus & threat protection tile (or the shield icon on the left menu bar) and then click Ransomware protection.

3. Under the Controlled folder access section, click Allow an app through Controlled folder access

4.Click Add an allowed app and follow the prompts to add apps.

source: https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-exploit-guard/customize-controlled-folders-exploit-guard#allow-specific-apps-to-make-changes-to-controlled-folders