I was able to make image and file backups of windows 8 pro but suddenly i receive the following error "access denied" I have tried other backup programs with the same result. In my ignorance it seems something is locking my C drive. what can it be?

Method 1: Check if the Volume Shadow Copy service is running.
a. Press Windows + R, type Services.msc . When the Services page is open, check for Volume Shadow Copy service.
b. Right click on Volume Shadow Copy service (VSS) and Check if it is started.
c. If it is stopped select Start or Restart.

Method 2: You may try giving permission to the backup drive in Windows Explorer and check if it helps:
a. Right click on the drive and select Properties.
b. Click on Edit button in Properties windows. Click ok to confirm UAC elevation request.
c. Select user/group from permission windows or click Add to add other user or group.
d. Now under Permission section check the rights which you want to grant i.e. check “Full Control” under the “Allow” column to assign full access rights control permissions.
(Generally we add ‘Everyone’ to given permission to all the users on the computer).
e. Edit the permission to Full Control for ‘Everyone’.
f. Click OK for changes to take effect and click ok again to exit from Properties window.
You may try giving permission to the backup drive in Windows Explorer and check if it helps:
a. Right click on the drive and select Properties.
b. Click on Edit button in Properties windows. Click ok to confirm UAC elevation request.
c. Select user/group from permission windows or click Add to add other user or group.
d. Now under Permission section check the rights which you want to grant i.e. check “Full Control” under the “Allow” column to assign full access rights control permissions.
(Generally we add ‘Everyone’ to given permission to all the users on the computer).
e. Edit the permission to Full Control for ‘Everyone’.
f. Click OK for changes to take effect and click ok again to exit from Properties window.