Hi and welcome to the forum
Some double fluorescent light units are wired up together and use 2 starters with 2 globes. If one of the starters or one of the globes doesn't work, neither light will come on. The starters are small white plastic cyclinders about 2 inches long x 1/2 inch diameter. You normally have to take the cover off the light to get to the starters but not always. Sometimes you can access the starters by removing the globes and the end of the starters stick out a little bit where the globes are.
There are 2 types of fluorescent starters. One is for a single light unit. The other is for a double light unit. You need to get the correct starters otherwise it won't help. They should have text on the side of the starters saying what they are.
When replacing globes on double light units, you should replace both globes and both starters at the same time because they wear out together. Try to get globes with a 6500K (K is for Kelvin) rating. These provide a clean white light with red and blue spectrums that closely resembles sunlight.
If you post a picture of the light unit we might be able to see the starters.
If you know the model we might be able to find an instruction sheet on it online.
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Alternatively, replace the light unit with a new LED light unit. They are generally cheaper to run and the more fancy models can have the different light spectrums come on at different times so you can have yellow in the morning, then add red and then blue, which resembles the sun coming up. In the evening you reverse that pattern so it's like the sun going down. The drawback to new LED light units is their price and you might be financially restricted and unable to afford one.
