Hi, i was drawing in Krita and i save my work, the next day that i tried to open it says that file not valid, please help, What´s going on?, How can i open the drawing? I really want to continue working on that drawing

When you're working on 'filename.kra' and you Save it, krita also makes a backup file of the initial file state called 'filename.kra~' in the same folder. The '~' on the end makes it a hidden file so your file explorer doesn't usually show it. If you tell your file explorer to show hidden files you should be able to see it and rename it to 'filename-previous.kra' (or whatever). Can you find the backup file?

If you're working on a large image for a long time it's a good idea to regularly Save so that the backup file contains a recent state of work.
Other useful methods, especially for complicated images, are to use Save Incremental Version and Save Incremental Backup to give you a development history record.

Try to open the .kra file as a Zip archive. If it cannot be opened in the archive manager, you will have to repair it. Or, try the backup file, as above.