I m trying to conceive since last 7 8 years I...

possible causes of IVF failing ?
generally speaking , IVF successiton rate isnt 100% , as it falls around 40% , so IVF failing is possible . but there may be other causes such as : 
1. female old age 
2. low quality eggs and sperms 
3. bad habits during pregnancy such as smoking and taking medication without doctor consultation
Can you do IVF again ?
yes you can , some people have tried IVF for 3 -4 times before it works 
General advices before start IVF :
1. have a healthy body and diet 
2. quit smoking and alcohol 
3. lose weight 
4. start taking suppliments including : folic acid and multi-vitamin  2 months before IVF 
5. avoid stress