Maybe your virtual disk got corrupted? Try making a backup of the VM in Proxmox and see if it fails with a I/O error, which would indicate a problem with your storage (or if it succeeds you have a safe copy of the VM before experimenting).
Boot the VM with an GParted Live CD or SystemRescue ISO and investigate the virtual disk? Do a boot repair? Do the same things you would do as if it was a physical machine instead of a VM.