Well, a PS/2 Model 25 got only a floppy drive, so if you deleted the DOS on your only system disk, you might be out of luck. If there is a hard disk, where the DOS files got deleted, then boot from a Floppy with DOS. This now gives two options (assuming the HD got detected and is accessible):
- Use the Undelete utility to recover the files or
- Copy the missing files from the FD onto the HD.
It all of course, depends on what got deleted.
Another way would be running the install disks. They should detect if the HD is already formatted before making any further damage and thus asks something like "Continue to replace existing DOS".
Either way, you need a DOS system floppy to do so.
Any yes, of course one could do it from BASIC, but that's a rather masterly task - not recommended for people that need to ask about :))