Have you tried cleaning it according to the product's documentation or another Fixya solution, such as the following:

http://www.fixya.com/support/t154553-ionic_pro_sizzles_crackles_when_turn

Because your unit apparently went bad while sitting unused and unplugged, I wonder if something crawled, flew or fell into it, perhaps shorting something out or just getting wedged in the collection grid.

Was the burning smell major (like actual smoke) or minor (just a whiff of something)? Did it smell electrical or otherwise? If you had a major burning smell, I would be sure to do any cleaning and future testing with the unit in a safe area such as a clutter-free workbench, i.e. a place where nothing is nearby to catch fire in the event of a flame or spark.

My ioinic breeze GP has sometimes had a very faint "burnt" smell, not particularly electrical, especially when I let it get very dirty. I consider this normal and never had any trouble due to it.

The product is no longer supported. Consumer Reports ran a report on ionizers and showed that the machine wasn't effective. Sharper Image sued and lost forcing the company into bankruptcy. The company recalled the units.

Heres an article last February about it

Here is an EPA article as well about ionizers you might be interested in