I have updated my iphone software to 6.1 on my 3GS and now my cellular data network is not available. Can this be fixed or can I restore an earlier version again?
Did you check your settings for your APN? Since iOS 6 it tends to clear the value on random times for some people. You can check this within Settings > General > Cellular > Cellular Data Network > APN.
You can only edit this setting if your carrier allows this. More info here: http://support.apple.com/kb/ht2283