If it's an older, T8 iBar, then as suggested by SteveUK try checking the starters (they're usually kept under screw in, water tight, caps). If it's a newer T5 unit however than it's most likely to be an electronic ballast with no starters, in which case the unit itself could well be fubar and need replacing.
You have my empathy, as we are in a similar boat. Our 11 ear old T8 bar is literally falling apart! We have more or less decided though (after considering DIY) to get some acrylic sheet cut to size to act as a condensation cover and try to get either a 4 tube T5 luminair or a HQI pendant off of ebay.
I refuse flatly to pay Juwel Â£75 for a bar that will only take special tubes, or Arcadia Â£80 for a bar that will take any T5 tube. Not when you can get a luminaire off Ebay for only slightly more or even less if you bid rather than using Buy Now.