Make sure your TAE buffer is fresh, make sure your DNA is not too concentrated. Don't use more than 2ul of loading dye per 5-7ul sample. Use EtBr 5ul in the gel casting and 30ul EtBr in the loading buffer. 1% agarose is usually a sufficient concentration for most runs, 1.5% may be too concentrated. 90v on the gel is a bit slower, but gives me better results. There are protocols for adding bleach to the gel and the gel buffer for rna runs to inhibit rnase.

Amy Johnson, thank you very much for the answer, my TAE buffer was actually fresh, and I usually add 1µl of the loading dye to 5µl of the sample. I usually do not use EtBr instead i use ROTH loading buffer which has been giving me results before this time. But this time, it is strange that i didnt even see the ladder

yeah if you don't see the ladder, something in the gel and buffer chemistry is wrong, or in one case my power supply was dying and making crazy stuff happen