This is your wrong "then put it in -20c until second day" you should place it in deep freeze or in liquid nitrogen until you convert it to cDNA  then you can store it in -20 as RNA mostly unstable and could be decomposed rapidly under -80.

The problem might be that you're running a DNA-agarose gel and trying to visualize RNA.  

We generally visualize RNA on either a TBE-Urea Denaturing Polyacrylamide Gel or on a Formaldehyde Agarose Gel (which is similar to the DNA gel but contains different buffers, formaldehyde and formamide).  I've attached a link for a formaldehyde gel protocol.  

You will indeed want to buy an RNA marker, but  at first you can just run your samples on the RNA formaldehyde gel and make sure you are getting bands.

Finally, RNA is very prone to degradation. You must use proper technique when working with RNA. This includes filter/barrier pipette tips, DEPC treated reagents, careful handling.  

http://openwetware.org/images/7/72/RNA_gels.pdf

As above, waiting 3-4 days before extraction,  and storage at -20 is not a good idea given the unstable nature of RNA

Also make sure ALL your  reagents buffers , water, tubes are RNAse free and make sure everything on your bench and pipettes are spayed with RNAseAway  when working with RNA.  Keep samples on ice while working and store RNA  -80 or below.

Also make sure you are not running the gels too long ( running your samples out of the gel)  before checking.

I agree with Dr. Labeed A. Al-Saad. Sample preservation before extraction is a very crucial step for this kind of studies that should be applied with great caution.

Some options will be use RNA later or isolate PBMCs and store them in -20 before RNA extraction. Hope it helps.

I had similar problems, but I shifted to paxgene tubes. The limitation with paxgene tube is that you would need their extraction kit. Trizol method doesn't work.

All the best. Please do update us if you find any other good options.