There are many different situations in which your SkyComputer may get deleted making you lose your data:
One Time Purchase: You will lose your SkyComputer if there is no SkyCredits consumed within 7 days.
Monthly Subscription: Your SkyComputer will NOT be deleted as long as you have an active subscription. You do not need to keep logging in to your SkyComputer to prevent the deletion. However, cancelling your subscription will cause a downgrade to the One Time Purchase status when your monthly plan expires and is not renewed. That will cause your SkyComputer deletion and decrease in storage from 500 GB to 200 GB.
If you miss your payment and you get downgraded to One Time Purchase, you will also lose your SkyComputer and all your data.
Changing Data Centers: You will also have to delete your SkyComputer before you can connect to a different data center.
When you upgrade from 200GB to 500GB you get to keep your SkyComputer and nothing is deleted.