A mistakenly deleted Photoshop document.

If it’s a traditional Photoshop document in your Windows file system, the answer is the same as it is for any other file:

• If it’s still in the Recycle Bin, drag it back out. 
• If it’s no longer in the Recycle Bin, retrieve it from your most recent backup. My backup software updates the backup every hour, so I rarely lose more than an hour of work. 
• If it is in a cloud-synced folder (such as OneDrive), and your cloud sync service saves document versions or holds deleted files for a while before permanently deleting them, see if it can be retrieved that way. 

If it’s a Photoshop Cloud Document, when it’s deleted it’s actually held for 30 days before permanent deletion, and during that time you can retrieve it if needed. In the Photoshop Home screen, click Deleted, then for the deleted file, click the ellipsis menu (…) and choose Restore.

Check the Recycle Bin.

Try Control + Z.

What operating system?
How was it deleted?