Can you let us know the gel concentration? If the gel concentration is not high enough, then the cDNA might diffuse into the gel, which appears as a single front. And please check whether the gel tank is short circuited or not.

An influence from external magnetic field may affect the run as well.

It looks to me as if the gel has run too fast and heated up to a point where the gel is not able to differentiate dna sizes (close to melting) so everything has run together. Gels should be run at about 5Volts/cm where the length in CM is measured between the electrodes. What voltage and current are you getting in your gels and did this gel feel hot when taken out of the buffer and was there condensation on the lid of the gel tank?

Malith Arambage Thank you for your suggestion. Last day I prepared 1% Agarose. This time I used 1.5% Agarose but the results were the same

Paul Rutland Thank you for the valuable suggestion, you were right last time my gel went off I run gel at 100V/100mV. This time I've used 80V/60mV.I got The band but still , like the previous result it is showing the same.

You could try running at 40v and use much less EtBr . The top band across all of the gel could be EtBr running in the gel and obscuring your dna signal