I was shooting at night and my flash was working perfectly, all of a sudden I tried taking another picture and it stopped working. I was using it on ETTL mode, the pictures looked underexposed with no signs of flash, then I tried using it in manual, what happened next was an overexposed picture because of the flash's light. Because I realized it was kinda working in Manual mode I set the ISO to 100 and a very fast shutter speed and it did the job, but this means that there is no way I can use my flash with the right exposure. I tried using my flash on my canon 60D and it works perfectly. Hope someone can give me an answer?
I had a common problem with my 5DII's where the hot shoe on the camera was coming slightly loose over time and the flash(es) I have would work intermittently on E-TTL. I've also had other friends with the same setup tell me the reverse, that the flash hot shoe was coming loose. The flash can still be triggered in Manual mode when E-TTL doesn't have a full connection because Manual has a set "flash" duration.
Some days everything would be fine, other days I had to run them on manual for an entire wedding. Its an easy DIY fix and there are a number of video's on youtube. Once I tightened everything (and checked it every month or two) I never had another problem.
If the camera is triggering the flash in Manual flash mode, then it should also be triggering it in E-TTL flash mode. My guess is that the Flash Exposure Compensation (FEC) setting accidentally got adjusted too low.
Another possible issue is that a highly reflective object was introduced to the scene. Possible items could be the highly reflective tape used on vehicles such as large trucks and buses or used on safety vests worn by public safety officials and construction workers. Since the E-TTL system is based on measuring the amount of light returned from a pre-flash, such objects can fool the system and cause it to severely underexpose.
Even when shooting manually, you can adjust the flash power down instead of reducing the ISO or narrowing the aperture. Since the flash duration is usually much shorter than typical shutter speeds used on low light the only effect of a faster shutter speed on flash will be if the shutter speed is shorter than the camera's sync speed. In that case only part of the sensor will be exposed when the light from the flash reaches the sensor, and the part(s) of the photo from the part(s) of the sensor covered by either(both) of the two shutter curtains at the time will be dark.