My holding made a loss of $500,000 on stock trading this week 
- the problem is I can’t figure out how or what happened. The bank account only shows transactions for the 24 hours.
Is there some way to find out, or may get a notification?
AFAIN no, but if you don’t remember selling stocks by yourself - maybe some company you owned stocks of went bankcrupt…
You mean I don’t get my money back? This is as real as it gets … 
Well, that depends on the company you owned shares in.
When it goes bunkrupt, all the money that is left is destributed among the shareholders, but sometimes there´s just nothing left to distribute.