'Big Time Rush' Star Kendall Schmidt -- Airline Lost $7,000 Worth of Gifts for Fans

Kendall Schmidt from "Big Time Rush" tried to do a nice thing for his fans in Europe ... but says he got screwed when the airlines lost his bag, and thousands of dollars worth of jewelry.

Schmidt -- BTR's lead singer on the TV show, and in the band -- is currently performing with his other band, Heffron Drive, on a European tour ... and wanted to give his fans some custom-made necklaces and bracelets.

According to our sources, Schmidt was flying from Orlando to Austria when the bag filled with the jewelry went missing.

We're told the luggage eventually resurfaced in Berlin, but the $7,000 worth of goodies had disappeared. We're told Schmidt filed a report with the airline ... in hopes of getting a cash reimbursement.

At least Schmidt was able to joke about it during his show in Vienna.