As Joe O suggests, one should never hit "Submit" without first doing the Ctl-A (select all), Ctl-C (copy) routine so that if things go awry one can Ctl-V (paste) without recomposing.
Oversimplifying greatly:
An error, when the site has gone offline and is NOT PRESENT, will usually result in your Back button looking on your own machine (i.e. reverting to working offline) to display what was up on the last previous screen from your Internet Temp folder. If you can re-connect, you can often proceed again in the forward direction.
(A good way to reconnect, for those of us stuck in the dial-up world, is just to open a NEW INSTANCE of the browser, leaving the screen you got with the Back button intact. When the connection is confirmed, the new browser instance can be closed and you can proceed from the backed-out original browser window. ... Sometimes.)
An error, when the site is present and accessible, usually will let you go back to the previous screen. This happens if your Submit drops a few bits, or clashes with other traffic on the server, or gets mangled in some other mysterious way. What you may have typed in that screen will be recovered from your Internet Temp folder just as above.
An error, when the site is "present" but NOT ACCESSIBLE, will usually upchuck and lose it all. The request for the previous page is "answered" with another error message which must be displayed instead of the previous page, which prevents your machine from resorting to the "offline" page in your Internet Temp folder. This is usually the case with the "503 Error" that's the subject of another current thread.