You can try meditating on what it is you have lost, that in my opinion can be the best way to go. If you still need more help I would suggest studying some herbs and crystals that help with memory. Then make a herbal bath surrounded by the crystals and meditate in there.
Are you talking about specific memories, or your ability of retention?
I hypothesize specific memories could be found through meditation and hypnotherapy.
I find I've increased my memory after opening my third eye (I found it has drastically improved my imagination and ability to visualize in my mind's eye, thus improving memory). Furthermore, I've found that positive affirmations can help memory, such as; "I remember everything I wish to remember","I have a great memory", etc.