How long have you been facing this issue? Is the internet working on your phone?
Let's try a reset first. Press and hold the power button and home button together until you see an apple logo. Now try to backup.
If that does not work, go to settings - general - reset - reset network settings. Now reconnect to your wifi and try backup. Let me know
Are you able to try from a different wifi connection? Also, do you have enough available space in cloud?
Sorry for the delay. I did not get an email that you had responded.
your iPhone backs up to the computer though, doesn't it?
I would suggest to backup again to the computer and then do a restore of the phone. After the restore, choose the latest backup to go into the phone. Once that is done, try backing up to iCloud.
Let me know if you find any difficulty.
No worries. Let me know if you come across any difficulty.