You could pick the lock, its a bug if you have the Fighters Guild DLC.

How do I fix the bug? is it still possible to buy the upgrades for my wizard tower?

The installation of the Fighter's Stronghold official plug-in changes the Mystic Emporium into a public area (with no owner), whereas normally the store and all of its contents are owned by Calindil, or if Wizard's Tower has been installed, a faction with Calindil and Aurelinwae as members. This has several consequences:

The entrance to the store is permanently locked. You can unlock the door (using spells or lockpicking), at which point entering the store is not considered to be trespassing (although the act of unlocking the door will still place a bounty on you). At 8 pm every night Calindil will relock the door; he will try to kick you out of the store if you are there at closing time (although after he has locked the door he doesn't care if you reenter). If you don't want to keep on unlocking the door every time, then get inside and unlock the door to the private quarters. This should solve the problem of the main door being locked during the day.
For Xbox 360 users, there is a work around that will let you get through entrance to the store without having to unlock it. The work around is described in the discussion page.
All of the items on display in the store can be freely taken, without being considered stealing. Even the Feather Shoes and Fortify Fatigue Pants become public property.
Calindil and Aurelinwae will barter with you, although still only between 8 am and 8 pm. Calindil will no longer sell you any of the items displayed in the store, but he will sell the items from his merchant chest (which include scrolls and soul gems).
Although the basemant and upstairs living quarters continue to be private areas, neither Aurelinwae nor Calindil will follow you if you enter one of these areas. Therefore you can brazenly walk in and steal everything in sight at midday with virtually no risk of being caught.
When both Battlehorn Castle and Frostcrag Spire are installed, this bug is influenced by the relative order in which the two mods are loaded. If Frostcrag Spire is loaded after Battlehorn Castle then the front door is permanently locked, but the items in the store are still owned by Calindil, and Aurelinwae will follow you around the store. The mod load order can only be changed on the PC (see Mod Reconciliation: Load Ordering).
PC Only This bug is fixed by the Unofficial Official Mods Patch.