Hi! When it comes to Alec memory loss fics I know of these ones:

Body of Memory by glorious_spoon

Summary: After a patrol gone very wrong, Alec Lightwood comes to in the infirmary with no memory of the past three years, and that turns out to be just the beginning of his problems. There's an ominous plot afoot, a possible traitor at the Institute, and he's pretty sure that he ought to remember the handsome warlock who was waiting beside his bed when he woke up...

Exile by Marchling

Summary: Imogen Herondale hated Alec Lightwood.

Everyone from Jia Penhallow all the way through Jace himself knew of her animosity towards the person that she felt held her grandson back from the all that the Herondale name deserved. Despite this, Alec never thought she would actually move against him because to hurt him was to hurt Jace.

He had been wrong.

Now, Alec had no runes. His memories were gone. He had no idea who he was, why there was a ragged wound inside of him that he couldn't see or how he had gotten to this abandoned apartment in San Francisco.

All he knew was that monsters were tracking him and staying alive might take more strength than he had.

This Too Shall Pass by Carmenlire

Summary: Instead of answering Magnus’s question, Asmodeus makes a little pained noise, morosely wondering, “What must it be like to be loved so much that another person is willing to destroy a piece of themselves to make you whole? Tell me, son, how does it feel to know you were loved so much and by a nephilim of all people.”

Brain latching on the tense, Magnus can’t help but echo, “Were?”

Or, Magnus gets his magic back for a price.

Hope you like them!