when this happens to me it is usually because my isp is fiddling with their dns servers and i have to reboot my dsl router to pick up the new configuration. not my computer. i don't know what the heck they are doing, blocking ports or moving servers or what. wish i did.
but the fix is not at the computer, betcha. probably could do a reset of router FROM computer, thru the web browser interface. might be interesting checking the Status page whilst there to see what dns it thinks it has...
if it had nice command language like cisco you could sent it precise dns flush type commands i bet but my little home office router can only be 'reset'...or 'Connected'.
when i had this problem on my little work network using dsl i solved it a couple ways. i called my phone company and talked to the router support guys and asked what did they think. we established i was using the latest firmware. we had my dsl line tested and it was fast fast fast. they asked me about any switches i had in my setup. i did have some switches, which made them recommend another brand router. they did not care what switched i had, only that i had them, wasn't that interesting?
following their suggestions, this is what i did. 1: i kept a log of the times i needed to reboot the router. piece of paper taped to router. this made it look like progress...
2. i put the dsl router on a battery backup (ups).
3. i enabled a 'auto reconnect' option that was in my router interface.
all this helped quite a lot. but not all the way.
my router firmware was already up to the latest.
it didn't really fix things all the way tho. i realized i was using same isp at home with no problems like this. i brought my cheapie cisco router from home and hooked it up and we were rock solid. try ebay for one....
so then for my workplace i bought the cheapest new cisco dsl router i could find. (they are not cheap) because get year of cisco tech support. oooooohhhh... and ...problem is solved