I would imagine that money in accounts may be lost, but shares/bonds in custody accounts would be returned to you (eventually).
We all know that banks and stockbrokers can go bust. What happens to their customers in that case?
Generally, the customers are insured up to some amount, for cash. For example, with interactive brokers, it is 100k $. If you have stock positions, you will be delivered the securities in paper form or they will be transferred to a broker of your liking.
If you have cash above 100k $, you might be in trouble for the rest of it.
Regards,
-Ip
From FT # Wealthy Americans seek refuge from Donald Trump in Swiss banks
Private bankers and investment managers report large increase in clients wanting to shift assets to Switzerland
Still safe to use IB and hold US Stock/ETF?
What is your opinion?
Correct, the deposit are safe up to 100k, however this projection comes from IMF, therefore I wouldn’t expect this to be settled in one or two days.
For the stocks and other financial instruments, I would personally say that if they stay with a traditional bank, it is just a matter of informing the custodian to transfer your securities to another one, for an online broker, I would say to check the contract as it appears that some of them also are lending your stocks around and there might be certain implications that could delay or put the process at risk.
It is always good to read the full T&C of each account opened and look up at the jurisdictions where your broker sits.
Just be careful where you put your money 
sometime we are too greedy and this could cost a higher cost in case of a major event.
I remember around 2011 when Cyprus (due to a massive exposure to the Hellenic debt of their financial intermediaries ) brokers were not allowed to let the clients withdraw their cash and I think, but not sure, also securities movements were restricted.