Does it look like all or most of your list bounced?
The first thing to check for is that there were no formatting errors on import. Sometimes .csv files need some extra love before they are imported. In some cases, you'll see the file try to cram a bunch of information into the email field instead of separating the columns out properly. If you go to Lists Tab > View List, you'll see really quickly if the email addresses are formatted correctly. This can happen for instance when an open quotation mark (") occurs in a spreadsheet. So if you have "help@mailchimp.com and then no closing quotation mark ("), the file may try to cram everything on that line into the first column. A google search on "remove quotation mark from excel" may yield some helpful results.
If you have had this list for a while and are starting to see high bounces, it's possible the list is starting to go stale.
If you feel the list could be stale, maybe it's time to reconfirm the inactive subscribers.
If this is the first time you have used this list and are seeing high bounces, it's possible the list was combined from multiple sources or contained old addresses.
We recommend reading the following to make sure your list is ok to continue using in MailChimp: http://www.mailchimp.com/abuse/list-okay.phtml