Well, if you still remember the key, you can use Tools -> Authentication Management -> Key Management-> Update Current Key to set the original key.
If you don't remember the key, probably you have to stop the DM Gateway and wait until key expires - if the expiration date was not set too long.
BTW, the key value was stored in DB instead of your file system, If you happen to backup the old DB, you can simply copy the string to new DB. Find the key in the dm_key table.
If you're out of luck with above options but have big number of units, you can try contacting the support team in your region for key decrytion. key1.key on your unit is required and usually this takes time for processing.