My pfaff 1475 cd has a problem. When I switch it on, the light goes on but the computer does not and I cant use it.

see if you can find out from this link.
How much does Pfaff 1475cd sewing machine Google Search

These instructions apply to PFAFF 1475 CD. They may also work for other machines.
If you have some advanced electronic repair skills and tools, check the following:
For 220 Volts models - there is a fuse next to the power input, make sure it is good. There is no such a fuse for 120 Volts model.
If the fuse is Okay, you have to get access to the main circuit board by removing the bottom cover.
Check the primary on the transformer for 120 V AC (220 V AC respectively). Check the secondary side for 5 V AC (luminescent display), 9 V AC (turns to 5 V DC by rectifier, 4700 micro farads capacitor and a 5 V regulator STI L387A) and 28 V AC (turns to 35 V DC by rectifier and 2200 micro farads capacitor.
Pay attention to two soldered round fuses in series with the secondary 9 V and 28 V - both should be good. On the big capacitors you should have 9 V DC and 35 V DC.
Common symptoms for bad small 22 micro farads capacitors next to the some of the IC's - there are 5 of these: These capacitors are in parallel with 5 V DC.
## All of the LEDs light up but the display is blank
## All of the LEDs light up, the display is blank, and after a minute no LEDs at all
## No LEDs nor display.
Replace ALL five 22 micro farads capacitors using a good desoldering pump or desoldering station!.
Enjoy your resurrected PFAFF 1475 CD machine!!!
I just did this procedure on my aunt's one (Made in West Germany around 1990).

here is a link for a manual--it should have the information in it that you need

Pfaff 1475 Creative CD Instruction Manual

Not sure if your model is the creative 1475. The closest I can find at a service website of Pfaff is 1473. Check if this is similar to your model and it shows where the balance wheel goes.
http://new.pfaff.com/SiteMedia/PFAFF/Products/Machines/Support-manuals/Creative_1473-manual-EN.pdf

Also see this Youtube video showing how to insert the bobbin after threading.
Pfaff Creative 1473 CD sewing machine in action

otherwise you will have to buy service manual for eBay

PFAFF CREATIVE 1475CD Sewing Machine Service Manual

it's possible that some part of the computer is or has gone out, it would be a good idea to have the machine checked.

where can I download pfaff 7550 menjal

http://sewingonline.co.uk/library/PFAFF-CREATIVE-1475-CD.html

Find a local Pfaff Dealer HERE and ask them about availability and price.

The typical rebuild on this board is done by replacing the 22 micor farrad caps. There are 3 or 5 of them depending if it is new or old style. Note: the power to the sew lamp goes straight from the motor to the lamp, the power to the control panel goes from the motor to the main board (base plate) then to the control panel. If you get the LED's on occasion then it is not the transformer or fuses in the base plate but simply the caps. Sometimes you can just scrape off the electrolytic gel that has oozed out of the caps off of their teminals, but replacing them is the best.
A Pfaff dealer can get a core credit for your board if you take it in. You shouldn't have to pay more than $200-$300 for a rebuilt base plate.