I changed my screen resolution on Windows XP Professional and
erroneously accepted a resolution that my monitor does not support.
Now I canīt see anything on my screen and donīt know how to return to
my previous resolution.
Any advise how to change screen resolution now?

Hello fdelucio-ga

What you need to do is to restart your computer in "VGA Mode".  To do
this following these instructions:

1) Shutdown/turn off your computer as you normally would.
2) Press the "On" switch on your computer to restart it and keep
tapping the F8 key to enter the Windows Boot Menu.
3) A menu screen should appear with the option to "Enable VGA Mode". 
Using your cursor keys highlight this option and press enter.
4) Your computer should continue booting in a very low resolution mode (640x480).
5) Upon reaching your desktop, right-click on the desktop and choose properties.
6) In the window that pops open select the "Settings" tab along the
top and alter your screen resolution as you would normally.

For further clarification, the windows boot menu screen, the one you
enter after pressing F8, can be seen on this page:
http://home.earthlink.net/~lreynol929/ruXP/Startup/ADVOPT.HTM

If you have any further questions on this subject or are still having
problems please ask for clarification and I will do my best to help.

Work exacltly as said.