Sent a lot of traffic to the site having all kinds of issues

Yesterday(Monday) we sent an email blast to over 6K people. We had many people reporting they couldnt access the page or download the audio that was on the page we sent them to. We spoke with our hosting and they upgraded us to a larger plan to handle all the requests we were getting. We thought late last night we were in the clear.

Today however, is more of the same problems, people cant access the page and/or the download on that page.

I am in a multisite environment and really having some doubts about our setup. Is it possible to turn multisite off and have separate websites? I have 3 subdomains and a main domain.

Or how do I check where the bottleneck is with wordpress? I am using namecheap hosting. I can see there are no errors at the server level. The only assumption I can make is that the WP database can not handle all the requests(actual people or plugins or or or or or)

Can anyone help me with this?