Lens cover stuck - Pentax Optio 750Z Digital Camera

Not sure if you mean the automatic lens cover, or the actual lens shutter, so here's fixes for both:
Stuck automatic lens cover
Stuck shutter

A stuck lens cover is a fairly common problem, but it is normally easy to fix. A single grain of sand jamming the cover mechanism is normally the culprit, and you want to try to dislodge it: 1) Try blowing lots of compressed air around the lens cover to clean the mechanism. Or use a hair dryer for a few seconds (don't want to heat up the camera). Use no heat mode if possible. 2) Slide a thin strip of paper between the lens cover shutters and the outer support ring. Work it around 360 degres to clear out any debris that may be in there. 3) With the lens barrel extended and pointed downward, tap the lens barrel with a pencil while you extend and retract the lens by powering the camera on and off. Repeat the above three steps several times. If repeated tries don't work (keeping in mind that it usually does), it then may be necessary to open the lens barrel to access the lens cover mechanism.

You can open the lens barrel up to fiix the covers, but only as a very last resort. The above procedures normally work if you try them enough. But for those particularly nasty stuck mechanisms, here's an example repair on a Canon Powershot A400 (do this at your own risk, and it will void your warranty):

http://camerarepair.blogspot.com/2007/11/repair-of-stuck-lens-covers.html

Please see the following article on automatic lens covers:

http://camerarepair.blogspot.com/2007/11/repair-of-stuck-lens-covers.html

Zoom error/ lens cover is stuck in open position

A stuck lens cover is a fairly common problem, but it is normally easy to fix. A single grain of sand jamming the cover mechanism is normally the culprit, and you want to try to dislodge it: 1) Try blowing compressed air around the lens cover to clean the mechanism. Or use a hair dryer for a few seconds (don't want to heat up the camera). Use no heat mode if possible. 2) Gently slide a thin strip of paper between the lens cover shutters and the outer support ring. Work it around 360 degres to clear out any debris that may be in there. 3) With the lens barrel extended and pointed downward, tap the lens barrel with a pencil while you extend and retract the lens by powering the camera on and off. Repeat the above three steps several times. If repeated tries don't work (keeping in mind that it usually does), it then may be necessary to open the lens barrel to access the lens cover mechanism.

You can open the lens barrel up to fiix the covers, but only as a very last resort. The above procedures normally work if you try them enough. But for those particularly nasty stuck mechanisms, here's an example repair on a Canon Powershot A400 (do this at your own risk, and it will void your warranty):

http://camerarepair.blogspot.com/2007/11/repair-of-stuck-lens-covers.html

This blog post outlines repair procedures for stuck lens covers. An A400 is used as an example. The A720 is more complex, but the base procedures are similar:

http://camerarepair.blogspot.com/2007/11/repair-of-stuck-lens-covers.html

You may have the same problem that i had. It does involve taking the camera apart. Not overly difficult - just fiddly.
Let me know and i'll talk you thro' it

I did the same repair...however, I found that those little wheels are completely optional. All they do is keep the lens cover from prying away from the camera, but if you're careful, the lens cover will work without them.

If you're careful this problem is a relatively easy fix for many digital cameras. The design of the lens cover mechanism is usually simple and very similar throughout camera brands. Here's an example fix for a Canon A400 with similar problems:

http://camerarepair.blogspot.com/2007/11/repair-of-stuck-lens-covers.html

the lens cover gets stuck sometimes. It just needs a gentle touch and it opens up. Only happened once to me. In my case it happened after my daughter held onto the camera with her finger on the lens cover.