I'd guess that bitcoin network in US and other isolated parts of the world would see a sharp drop in hash rate, but the blocks would still be generated... as the result, when communication is re-establishes itself, would we have many separate chains/networks?
Continuous connectivity is required to mine and is required by an exchange, merchant and individuals who trade and need to protect against double spending.
As D&T explains the Internet does not have regions that get isolated from each other as you describe, but for the sake of argument let's assume that can happen.
So region A has 10 Thash/s of capacity and the rest of the regions that are not connected to (i.e., cannot route to) A have a total of 15 Thash, but none larger than 5 Thash/s and none have any connection anywhere to each other nor to region A.
The first thing that will be noticed by all participants everywhere is that blocks are taking about 25 minutes (region A) to an hour (other regions) to solve instead of the normal 10 minutes. Since those in any one region don't know whether their region will emerge as having the most hashing capacity, those merchants, exchanges, individuals etc. might be wise to stop accepting payment in bitcoins from untrusted parties until a better assessment can be ascertained.
What will likely happen is at least one node in each the other disconnected regions will establish connectivity with a node in region A (even if it is a dial-up connection) and there are then no more disconnected regions (as one node in a specific region can successfully relay all blocks and transactions thus joining the formerly disconnected regions with region A.)
So can wide outages disrupt the ability to transact with bitcoin? Sure. Could bitcoins be lost due to a 51% attack during such an outage? Well, bitcoins aren't lost unless there was fraud intentionally perpetrated by a party with 51%. There's probably even less chance of success because there would be a heightened sense of awareness. i.e., an exchange might suspend all outgoing bank wires and withdrawals until the communications issues are resolved.
Remember, that if there's no connectivity, there's no connectivity -- so there is also no VISA, Mastercard, Bank (ATM), Gmail, Facebook, Salesforce.com, IRS.gov, Wikipedia, Netflix, etc. There would be pretty steady progress in re-establishing connectivity is nearly every instance.
Here's a related thread on the subject of what is available for an isolated region:
Desert island economy on Bitcoin without being connected to the internet? -
https://bitcointalksearch.org/topic/desert-island-economy-on-bitcoin-without-being-connected-to-the-internet-106302