If the plastic that cracked is the only support across the top, it needs to be replaced. Those cross braces hold the front and rear glass from bowing out, then breaking from the internal water pressure. If that is a decorative plastic fascia that is separate from another support setup that is still intact, then it may be only cosmetic. Putting the tank weight on a decorative assembly that is not configured to hold the weight (the tank was upside down afterall) could cause the breaks, particularly if the plastic is older, which I assume it is if you are siliconing.
If it is not a decorative piece then the concern would be why did it crack in the first place? Something must have flexed which leads to the stand being questionable. Perhaps the stand surface is not flat.