You can try getting a tub of warm water and putting your SAK in and opening and closing the blades a few times. Once you work them free, lubricate with WD-40 and see how it goes. You may need to repeat the cycle a few times.

SAKs are very hardy and if it still doesn't work, you can send it back to the dealer - they will fix it up for you. Victorinox has some outstanding aftersales service.

Good luck!