Ghosted? PI of >1 year not responding to emails for new mentor? I'm currently an undergrad who has been working at this lab for 1 year. In the beginning, I was pretty slow with research and I still make mistakes sometimes. For instance, it took me 4 months to get a western to properly work and 1 month to properly clone something. I'm not as bright as the other interns, but I learned a lot this 1 year about techniques and scientific knowledge. Granted, I don't know if I'm 'competent' enough for research. 

Anyways, my research mentor (post-doc) left for a new position, and now I'm pretty much without a mentor. I emailed my PI for a new mentor assignment if possible a week ago, no response. My former mentor emails my PI and gets no response. Now, I decide to follow-up with him yesterday - still no response. So in total, 3 emails (2 from me, 1 from my former mentor) sent to my PI about this topic in the last week. 

I'm not the closest to my PI. He seems always busy and I haven't spoken to him very often. However, it's unlike him to take a very long time to respond to emails, as he usually responds within 1-2 days. 

Does this mean I've been ghosted by my PI/given a boot from lab? My former mentor told me that my PI would be willing to let me continue working, but I don't know if he was just saying that.  

I'm not sure if I should reach out to other post-docs/grad-students in the lab and ask them if they can be my new supervisor mentor. So far, I haven't had much work to do so I've been staying back home and reading papers the lab has published/to see if any of interest. The other people in the lab familiar with my name; I hope they're not thinking about how stupid I am since I did make lots of dumb mistakes. I know the lab manager doesn't like me that much (one time I was giving a lab presentation and she was shaking her head), and usually she's the de facto person people turn for conflicts such as these.

What should I do?  I've been in a state of rumination about this for a long, long time. I'm really interested in the work of this lab.
Go and visit your PI. Knock on their door when they're in, and ask them everything above.

You write in a comment above that you feel feel "kind of scared/awkward doing that since he has a very cold, direct manner". Being scared should not be a deal breaker. Courage comes when you overcome fear! It's much more admirable to face what you're afraid of than to hide from it. Nothing critical will happen to you, and to have done something like this before is a great help if you have to do it again in the future, in less protected environments. As for how others don't knock without a meeting, you could knock and start with "are you free?" If he says yes then you're good, if not then you can arrange a meeting.

Finally you write that you don't know what to say or how you'd react to a rejection, which shouldn't be a problem. For the first, write down everything you want to know, and rehearse it beforehand. For the latter, if you are rejected, you're going to know in the near future anyway, and you might as well know now so you can make plans for what to do next.

It's better to be addressing your problems than to sit around waiting for the PI to respond. Go and visit your PI.