Rose balling is a condition where the flower bud develops normally and swells to full size but the petals fail to open.
The bud often feels soft and slimy to the touch and the outer petals turn brown.
Eventually the whole ball of fused (cocooned) petals either withers, or drops off in one piece.
This is generally caused by cool wet weather coupled with shade where petals have little chance of drying.
Early-flowering, thin petalled and heavy double cultivars are particularly prone to this disfiguring problem.
The cure is more prevention rather than cure i.e. it is important when planting out to choose an open site free of shade.
The prime consideration is protection from rain while allowing natural drying by wind and sun to continue.
If this is not within your control and you are an ardent grower or exhibitor, you might consider constructing a temporary shelter over the whole plant/s or place a small cone / umbrella over individual blooms.