Q: How do I fix chocolate that has seized (hardened)while attempting to melt it?
A: The Nestle Company's response was as follows....

Need to rescue your seized morsels, chunks or baking bars? The Nestlé Test Kitchens have developed procedures for CORRECTING SEIZED NESTLÉ TOLL HOUSE MORSELS, CHUNKS AND BAKING BARS. It provides the proper ratio of oil or shortening that may be added to your product to restore its texture.

To correct SEIZE morsels and baking bars, follow the instructions below.

Product............................Amt. of Vegetable, Canola, or Corn Oil / Shortening
Semi-Sweet Morsels and Mini Morsels 1 cup (6 oz.) 1 Tbsp. 1 Tbsp.
Semi-Sweet Chocolate Chunks 1 cup (6 oz.) 1 Tbsp. 1 Tbsp.
Milk Chocolate Morsels 1 cup (6 oz.) 1 Tbsp. 2 Tbsp.
Butterscotch Morsels 1 cup (6 oz.) 2 Tbsp. 2 Tbsp.
Premier White Morsels 1 cup (6 oz.) 1 Tbsp. 1 Tbsp.
Peanut Butter & Milk Chocolate Morsels 1 cup (6 oz.) 1 Tbsp. Not recommended

Semi-Sweet and Unsweetened Baking Bars 4 bars (8 oz.) 2 Tbsp. 2 Tbsp.
Premier White Baking Bars 3 bars (6 oz.) 2 Tbsp. 2 Tbsp.
Note: For best results, we recommend the use of the above oils and vegetable shortening with all of our products. Butter, spreads, water or milk are not recommended.

Method:

To SEIZED amounts of product add recommended measures of oil or vegetable shortening. Blend well.
If needed, add up to 1 teaspoon more oil or shortening to achieve a smooth workable product.
Use corrected chocolate as intended.