so if you loaded a defined amount of total RNA (quantified by measuring OD260, and it s not too less ;-) on your agarose gel you should see 18- and 28S ribosomal RNA bands. if it is not so, check your RNA concentration again. if you can reproduce your former amount of RNA it might be heavily degraded so that it is not visible after a long run:( i would check another method of RNA-preparation with focus on protecting RNA from degradation. use RNA protecting agents whenever possible. ambion (www.ambion.com) got fine reagents for this application.