I can not put memory card inside the camera, this card is ritzpix 2GB. Can u please tell me what can be the problem?thank you

Your problem is that you are not using a suitable memory card for the D90. 2Gb SD cards have read/write speeds that are much too slow to work with quality DSLR's. Let me explain. When you take a picture with the D90, the data generated first goes into a small capacity buffer memory. The purpose of the buffer is to act as a conduit to your SD memory card and retain picture data until all of the data is safely written to the SD card. If your SD card can accept data as quickly as the buffer can pass it to the SD card, there is no problem. But, If the SD card is not capable of accepting the data quickly enough, the buffer will start to fill up since data is coming in faster than it is going out. If you take pictures in quick succession with a slow SD card or if you shoot video with a slow SD card, the buffer will overflow; i.e., fill to its capacity and become incapable of accepting more data until its capacity returns as the SD card accepts the data. During that time, the camera will prevent you from taking any more still photographs. You will push the shutter release and nothing will happen. If you are shooting video, the camera will continue to work, but the sound will loose sync with the picture.

For your D90 to function properly under all conditions, you must have an SDHC card that is rated at 45Mbs or faster. When I first bought my D90 several years back, 35Mbs was the fastest card made and I had the problems I described to you above. When Sandisk brought out the Sandisk Pro card with a 45Mbs speed my problems melted away. I should also ad that 2Gb is much too low of a capacity for the D90. It will fill up very quickly if you shoot Raw or fine quality jpeg files. You need a minimum of 8Gb.