Juwel light bars are notorious for problems. A dodgy bulb/tube can make the other one fail too even though it's ok.
Ok, replacing both bulbs is expensive so if you have a friendly LFS try asking (not on a busy Saturday!) if you can try your bulbs, one at a time, in one of their display tanks to test them. Of course being prepared to purchase new ones from them if they prove to be blown.
If, as I suspect, the bulbs are ok then it will be the ballast. You then have 3 further options. Option one, purchase a new Juwel light bar (£80 plus) or try getting a second hand one (testing it first of course). Option 2, follow my pinned post at the head of this section in replacing the ballast (around £10). Or Option 3, purchase a halide light......great but loads of dosh!