Before summarizing Sonnet 30, let's translate the tricky bits into contemporary English; this will make finding the main idea and summarizing the sonnet much easier.
My beloved one is like to ice, and I to fire:
Why is it then that this her cold so great
Is not dissolv'd through my so hot desire,
But harder [her icy heart] grows the more I her entreat?
Or why is it that my exceeding heat
Is not dampened by her heart-frozen cold,
Instead I burn much more in boiling sweat,
And feel my flames augmented manifold?
What more miraculous thing may be told,
Than that fire, which all things melts, should harden ice,
And ice, which is congealed by senseless cold,
Should kindle [and increase] fire by wonder causing devise?
Such is the power of love in gentle mind,
That it can alter all the course of life.
The main idea of this sonnet may be approached from the perspective of the metaphor and from the perspective of the interpretation of the metaphor. Firstly, the metaphor compares the beloved's (Elizabeth Boyle) unloving heart to ice and the poetic speaker's (Spenser himself) loving heart to fire: Elizabeth continually rejected Spenser as he was much older than she and a widower. The main idea of the metaphor is a double one. The first part is that it is a wonder how ice, which is turned to water by fire, can be made more hardened and icy by the presence of love's fire. The second part is that fire, which is quenched (put out) by melted ice, or water, can be made to blaze more brightly and hotly by an icy unloving heart.
The main idea that emerges after interpreting the metaphor is that Spenser wonders aloud in the sonnet how it can be possible that his burning love for Elizabeth doesn't warm her heart and cause her to love him, while at the same wondering how it is possible that her cold unyielding heart doesn't completely dampen his love for her but instead makes his love grow deeper and stronger. It is a paradox: love should kindle love but doesn't; an unloving heart should quench love but doesn't.
The summary is easier to see now. Spenser is complaining, in this sonnet "complaynt," that the object of his love, who does not love him back, doesn't warm up to him as he entreats her to accept his love but instead gets more and more unloving and unyielding, like ice and fire in a paradoxical reverse. He continues to complain that his love for her is not cooled off by her rejection but instead grows deeper, stronger and hotter; this continues the ice and fire paradox. Then, he complains that his reaction and hers are contrary to nature: love offered should kindle love returned or love rejected should foster love abandoned, not the reverse. He resolves the paradox by claiming this is the power of love: it simply can alter the laws of nature:
Such is the powre of love in gentle mind,
That it can alter all the course of kynd.