I've worked in these kind of roles and this has happened to me on occasion - it's very stressful when your boss leaves under normal circumstances (retirement or promotion) but yours sounds to be unusual and possibly contentious.
In one situation, where my boss left quickly, no one said anything to me for three months! I just kept coming in - her replacement had his own PA and I tried to find bits of pieces of work to keep me going. I think they expected that I'd get bored and leave but it was at the time of the crash and there were no other jobs. Eventually HR asked if I'd be interested in another roll on the same salary which i took.
In another situation, my boss retired and her replacement was awful! Compulsive liar, hated me on site, lied about me, tried to get me fired (fortunately she wasn't the cleverest and I had enough evidence to show that she was making stuff up about me). I waited it out through humiliation (she demoted me to data entry clerk) as I knew voluntary redundancies were coming up soon and I left then. But otherwise I would have made a formal complaint.
In my current job, my former boss was sidelined and I've been moved to another person at the same level - it was awkward as they wanted me to go to a dept that wouldn't have suited me but I pushed back and got the role I wanted. But I was lucky in that there were options. I was prepared to leave if I didn't get the role I thought I was best suited too - easier to to that in improved economy.
Advice wise, don't panic! But don't presume anyone will tell you anything either - they may just leave you sitting there, expecting you to leave. But they can't fire you - they may offer you a deal to leave, a couple of months salary, if they believe your loyalty was to him and they are trying to wipe the slate clean. Try not to take that personally if it's the case. Smile, shake their hands and take their cheque and move on.
Best of luck - it's so stressful because there's no one to share it with, other people don't get it. As a PA your role is tied up in your bosses and others won't understand that.