Hello Saloni,
As Bob suggested, you should first delete it from your sand box using force.com IDE.
Below are the steps:
1) From Force.com IDE, select the class that you want to delete (Look for .xml extension )
2) Open that .xml version of your class and change the Status tag to Deleted.
3) Right click and save the file.
4) Now select “SRC” folder and right click—>Select Force.com—> select Deploy to server
5) Log into your production org and follow the steps.
Note: if you have any test class for the class that you are deleting make sure you do the same like above and then select those 2 classes for final deployment.
Hope it helps.