Your wallet can easily be restored using your private key, if you haven't yet backed it up, then please read here on how to do that.
You can also restore your Stellar wallet created elsewhere; such as from Lobstr, Albedo, etc. using the private key from those.
If you have freshly installed StashApp, when you open the app you will be greeted with the following screen:
Type or paste your private key in the field Import private key and tap on the arrow, as in the image above. Then wait for StashApp to complete the download of your wallet data from the blockchain.
Your wallet can also be restored using Wallet Guardians if you have previously set this up in StashApp.