That normally indicates that the input / source / video selector on the TV got changed, use the TV's remote to change the input / source /s video back to the proper input. Since the cables have been checked and it's still not working, then unplug both the TV and cable box AC power for a minute, power up the TV first then the cable box a minute later and see if that helps.
The TV stopped working and only shows a blue screen with the words "no signal". All connections have been checked.
Just to add to what JayInAlg already posted above:
No signal usually means that the TV set is on the incorrect video input.
Check to see how your cable box is connected to the TV set.
If connected via RF-coaxial cable, the TV should be on channel 3 or 4.
If connected via HDMI be sure that HDMI is selected on your TV set (there may be an HDMI 2, HDMI 3, etc. input on your TV set, check them all).
If you have X1, be sure that you are connected to the HDMI output on the X1 cable box (not one of the the HDMI inputs).
If you are connected via component cables (red, blue, green) be sure that your Component is selected as the input on your TV set
If your TV is connected via composite cables, (red, white and yellow) check that your TV is selected to Video or Game .
Also be sure that your cable box is turned on.