You got the job offer and then they withdraw it.  No explanation, no reason, no nothing.  So what happened?  It could be economics but it could also be some information that the organization gets on you that puts you in a bad light.  For example the degree you said you had in your application is not actually complete - although it probably will be next year - but the employer doesn't like that.  You told a fib and you got found out.  Or the company finds out that you were a pain in your old job and your boss didn't like you.  The company contacted your old employer and found out you were not Mr or Ms Popular.  Or you send a thank you note which has a bunch of typos in it and the company decide you blew it.

So what can you do?
You can ask.  You can ask to find out more information on why the offer was reduced.  If you persevere then you may get an answer.  If you know then you can either explain or learn for the future.  
If you know there is a problem - e.g. your boss will say bad stuff - then warn your new employer and explain why - and give some references who will say positive stuff.