Hello Pranita, more information would help folks troubleshoot. Could you kindly attach your protocol? From what I can glean, you PCR amplified both the promoter region from gDNA and the vector and then attempted blunt-end ligation followed by transformation and colony PCR? Were your primers phosphorylated? Following PCR, did you digest the original reaction with DpnI and then do a PCR cleanup prior to ligation? What vector to insert ratio did you use and what incubation time and temperature did you use? Without more information it is difficult to pinpoint the problem. Good luck!