There are several species of aloe and some naturally have bare stems are they grow.
If you don't have one of those species, It could be that too many of the outer leaves are dying.
Outer leaves die for many reasons: over or under-watering, frost, disease, insects or too much sunlight. Try to figure out how to make you plant's environment more ideal.
When a stem gets bare, you can cut off the top and root it. Here is one of many helpful webpages: https://gardening.stackexchange.com/questions/31419/can-i-cut-the-top-off-a-drooping-aloe-plant/3145...