I’m a newbie. I placed a security deposit but was then delayed before placing the offer to buy BTC. When I returned my deposit is not in my wallet and has gone to another address. I have the page showing the transaction in mempool. How do I get it back?
Welcome to Bisq 
Sounds like you have created an offer.
If so, you have not lost anything!
The transaction you see if miner fee for the security deposit being placed in a multi-sig.
Go to Portfolio > My open offers > you can edit or delete your offer there.
If you delete it you get your funds back.
Hope that helps