I haven't used my machine in years It runs but very slowly and the side button is stiff. What should i do

Remove the plate & give the jaws a little drizzle amount of oil & let that seep in then gently hand crank the machine until the jaws start moving, using cotton buds to apply the oil is a better way of applying the oil, but don't over do the oil or the fabrics will be ruined upon using it again, wipe of the excess from the jaws & run scrap fabric through until no oil residue is visible at all... I hope this helps....

I have resurrected several older Berninas with lots of patience and elbow grease. Here's how:

Wheel on right side that controls the needle up and down is very tight...

Bernina 830 Record oiling points:

although the machine is computerised, the moving parts are still mechanical and may be stiff, try a lubricating spray

If you turn the handwheel by hand is it stiff?

Try unpluging it from the power. It may need to resent . Try a basic straght stitch and see what happens. Then go back to your Ziz zag stitch.

Use your ruler and measure your button. Then transfer those measurements to your garment.
Use chalk to mark the center cut line of your button hole. Mark the end or stop point of the button hole per the measurement.
Practice on a scrap of the same fabric before you begin on your garment.
Set your machine the satin stitch zig zag at a fairly narrow width.
Begin satin on one side of the center line of the mark. Slowly stitch down that side until you get to the end. STOP. Widen your satin stitch to the max. sew about 1/8" and stop with the needle down on the opposite side to your first line of stitching. Turn the fabric, narrow the stitch to match original satin stitch. Sew slowly down the other side of the center mark. STOP, lengthen the stitch, sew 1/8" then do a couple back stitches.

That's one way. Another older way is to sew 2 or tree straight stitch lines around the center line. Remove then use a hand rendered button hole stitch after you cut the hole.

try adjusting the top thread tension one number or letter higher
and sew on a scrap piece of fabric till the stitches look normal.

since the machine has been setting for 20 years, it needs to be serviced
the old grease & oil have probably turned the gunk. the servicing can also tell if any parts are damaged

There is no speed adjust, but 1st you must know the source of the slow down. Remove the belt from the motor pulley and run it, if it goes fast try turning the hand wheel if it is stiff the machine needs clean and oil. If it turns freely you motor may be dirty or just slow. If dirty, sometimes you can without the blet attached run it at full speed for 10 to 15 minutes. It should get faster as you run it. If not motor is weak or slow needs replace. Just replacement motor as genuine probably no longer available.

Open the front cover and raise presser foot to release all threads. Depress the machine lock button firmly with your left index finger. At the same time manually rotate the machine hand wheel slowly forward until he button "snaps" into the locked position and the lock button release lever moves to the far left.