Why am I being taxed on my HSA for which $5000 was contributed in previous years? All of it was used in 2017 as well as my contribution for 2017.

A 1099-SA shows the amount distributed from your Health Savings Account during the year and is included as income until you enter the information on how much you actually spent for medical expenses.

After you enter the associated medical expenses, this should resolve.  Please see the TurboTax FAQ below for how to accomplish this:

https://ttlc.intuit.com/questions/3894040-where-do-i-enter-the-amount-for-1099-sa