Computer will not find DNS automatically...

Midway through a browsing session my computer suddenly stopped bringing in pages. The connection status of my DSL connection showed as 'connected' but I kept getting an error message to advise that the URL could not be found.
I have two computers which connect through a switch to my router, only one computer would not work and swapping the cables between the two computers at the switch had no effect on the problem computer.
A call to my ISP revealed that the problem computer would not find the DNS automatically, but when the numbers were entered manually it had no difficulty finding the pages and seemed to be working normally. I am now working on the fixed DNS numbers that the ISP provided.
I would appreciate any suggestions as to why the automatic option suddenly failed and if possible, how to correct the situation.

Thanks!
	DD
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